This application claims the benefit of Japanese Patent applications Nos. 2000-269193, 2000-270687 and 2001-252345 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an eyepiece lens and, more particularly, to an eyepiece lens most suitably for the use in a camera, etc.
2. Related Background Art
In an eyepiece lens for observing an image formed by an objective lens through an erect system, it is necessary to enlarge the erect system in order to obtain a higher magnification since a light beam spreads. Then, since the length of an optical path for the light beam is prolonged, the focal length of the eyepiece lens has to be increased in proportion thereto. As a result, it is impossible to achieve a higher magnification. Particularly, an eyepiece lens having a long eye relief (the distance from the eyepiece lens to the eyepoint) has this tendency conspicuously.
Moreover, as an eyepiece lens for an observing optical system of a camera, or the like, there is known a lens which is comprised of three groups including a positive lens group, a negative lens group and another positive lens group, so as to move an arbitrary lens group along the optical to correct the diopter. As such an eyepiece lens, various lenses have been proposed including one which is disclosed in Japanese Patent Post-Examination Publication No. 61-19968.
With the recent electronization of cameras, etc., various display devices such as an image pick-up device or a monitor are increasingly mounted on a camera body. As a result, the camera body has to be enlarged, and an optical finder is required to have a greater distance between the focal surface and the eyepoint correspondingly, compared with conventional ones. Also, an image formation area on an image pick-up device is made smaller than one frame of a conventional silver halide film, so that a finder magnification is also required to be greater. Further, the eyepiece lens is required to additionally have a diopter correcting function, and the like, in order to cope with the visual acuity of any photographer.
The present invention has been contrived taking the above inconveniences into consideration, and an object of the invention is to provide an eyepiece lens having an excellent aberration performance with a sufficient eye relief length and high magnification, and preferably, an eyepiece lens further having a diopter adjusting function.
In order to solve the above problems, there is provided an eyepiece lens for observing an image formed by an objective lens through an erect system, at least comprising a first lens group having a positive refracting power and a second lens group having a negative refracting power in the order from the side of an observer, and satisfying the following condition:
xe2x88x923 less than SF1 less than xe2x88x921xe2x80x83xe2x80x83(1)
where SF1 indicates a shape factor of the first lens group, and SF indicates a shape factor of the lens group, which is defined by:
SF=(Rs+RE)/(Rsxe2x88x92RE),
where Rs indicates the radius of curvature of a lens surface closest to the object side and RE indicates the radius of curvature of a lens surface closest to the observer side.
According to the present invention, with the above structure, it is possible to move the principal point of the entire eyepiece lens to the observer side, and to prevent a prolongation of the focal length of the eyepiece lens which may be caused by the prolongation of the optical path length of the erect system, thereby raising the magnification. It is also possible to secure a sufficient eye relief length since this eyepiece lens has a retro-focus type layout which has positive and negative refracting powers when seen from the object side toward the observer side.
Also according to the present invention, it is preferable to satisfy the following condition:
xe2x88x923 less than SF1 less than xe2x88x921xe2x80x83xe2x80x83(1)
Above the upper limit of the condition (1), the lens is not preferable for improving the finder magnification. On the contrary, below the lower limit of the condition (1), it is difficult to correct a distortion and a coma. This case is not preferable also in terms of manufacturing since a fluctuation in aberration becomes great due to an error such as eccentricity. Accordingly, it is practically desirable to satisfy the condition (1) that xe2x88x923 less than SF1 less than xe2x88x921
In this case, the layout of the second lens group and the first lens group is of a retro-focal type having the negative and positive refracting powers when seen from the object side to the observer side, so that a sufficient length for the eye relief can be secured.
According to the present invention, it is referable to adjust the diopter by moving the first lens group having the positive refracting power or a lens component having a positive refracting power in the first lens group, along the optical axis. In this manner, it is possible to adjust the diopter while maintaining a corrected aberration state, and particularly, excellent distortion. It is also possible to adjust the diopter by moving a lens component having a negative refracting power in the second lens group along the optical axis. In this case, a lens closer to the focal surface has a greater fluctuation in aberration when the diopter is changed. As a result, it is preferable to adjust the diopter by moving a lens component which is positioned on the observer side, in the second lens group.
Also, according to the present invention, it is preferable to satisfy, in addition to the above condition (1), the following condition (2):
1 less than SF2 less than 6 xe2x80x83xe2x80x83(2)
where SF2 indicates the shape factor of the second lens group.
If the condition (2) is satisfied and the second lens group is formed in a meniscus shape concave on the observer side, an excellent aberration state can be securely obtained. Also, since the principal point of the entire eyepiece lens is moved to the image formation surface side, a high magnification can be achieved. Below the lower limit of the condition (2), an astigmatism or a coma which is generated in the first lens group satisfying the condition (1) can not be sufficiently corrected. On the contrary, above the upper limit of the condition (2), it is difficult to correct a distortion. As a result, it is preferable to satisfy the condition (2) that 1 less than SF2 less than 6.
Further, according to the present invention, in addition to satisfy the condition (1), it is preferable to constitute the second lens group having the negative refracting power with a lens group having a negative refracting power and a lens group having a positive refracting power in this order from the observer side. Then, it is possible to conduct excellent aberration correction while maintaining a wide distance between the lens group having the positive refracting power and the lens group having the negative refracting power in the second lens group. It is also possible to maintain a long distance from the focal surface to the eyepoint to cope with enlargement of a camera size while aiming cost reduction of the glass material.
It is also preferable to satisfy the following condition (3):
xe2x88x922 less than SF2p less than xe2x88x921xe2x80x83xe2x80x83(3)
where SF2p indicates the shape factor of the positive lens in the second lens group. When the positive lens group in the second lens group is formed into a meniscus form which is concave on the observer side, the principal point of the entire eyepiece lens is moved to the imaging surface side. By the use of this phenomenon, the focal length of the entire eyepiece lens can be reduced to thereby raise the magnification. Above the upper limit of the condition (3), it is difficult to improve the finder magnification. On the contrary, below the lower limit of the condition (3), it is difficult to correct a distortion and a coma. Accordingly, it is preferable to satisfy the condition (3) that xe2x88x922  less than SF2p less than xe2x88x921.
Also, according to the present invention, it is preferable to satisfy the following condition (5):
Dx/fexe2x89xa70.08xe2x80x83xe2x80x83(5)
where Dx indicates the thickness of a lens group, and fe indicates the focal length of the eyepiece lens when the dopier is xe2x88x921, respectively.
If the condition (5) is satisfied, an excellent aberration state can be obtained. Below the lower limit of the condition (5), a coma can not be sufficiently corrected.
The unit of diopter is expressed in xe2x80x9cdptxe2x80x9d in the following description. The diopter X [dpt] indicates a state in which an image obtained by the eyepiece lens can be formed at a position of 1/X [m] on the optical axis from the eyepoint (the sign is negative when the image is formed closer to the object than the eyepoint).
By satisfying such condition, it is possible to increase the distance between the erect system and the eyepoint by making at least one of the lens groups to be thicker to prolong the optical path length while enhancing the magnification.
Particularly, as to the first lens group having the positive refracting power, if the concave surface on the observer side is moved backward to make the lens group thicker, the distance between the erect system and the eyepoint can be wider. In addition, the convex surface on the image formation surface side which has a strong refracting power because of the meniscus shape satisfying the condition (1) is not moved. For this reason, the focal length of the first lens group can be effectively reduced while maintaining an excellent aberration state. In the similar manner, as to the lens group having the positive refracting power in the second lens group satisfying the meniscus shape of the condition (3), it is possible to increase the distance between the erect system and eyepoint while maintaining the excellent aberration state by making the lens group thicker by retreating the concave surface on the observer side. In addition, as for the lens group having the negative refracting power, if the lens group is made thicker by moving forward the surface on the image formation surface side, the concave surface on the observer side which has a strong refracting power because of the shape satisfying the condition (2) is not moved. As a result, it is possible to increase the distance between the erect system and the eyepoint while maintaining an excellent aberration state.
Further, it is desirable to constitute at least of the first lens group satisfying the condition (1) and the second lens group satisfying the condition (2) with a single cemented lens. In this case, a longitudinal chromatic aberration and a chromatic aberration of magnification can be easily corrected if the above cemented lens is formed of glasses having different dispersions which satisfy the following condition:
|xcexdaxe2x88x92xcexdb| greater than 8
where xcexda and xcexdb respectively indicate the average rates of dispersion of the glass materials for constituting the cemented lens. It is also possible to reduce the processing cost of a thick lens by constituting the cemented lens with lens components of the same glass.
In addition, if the first lens group becomes thicker, it is difficult to adjust the diopter by moving the first lens group along the optical axis. Then, according to the present invention, it is desirable to constitute the second lens group with a lens group having a negative refracting power and a lens group having a positive refracting power in this order from the observer side, and to satisfy the following condition (4):
0.5 less than |f2n|/f1 less than 1xe2x80x83xe2x80x83(4)
where f2n indicates the focal length of the negative lens group in the second lens group, and f1 indicates the focal length of the first lens group, respectively.
By satisfying the above condition to provide the negative lens group in the second lens group with the strong refracting power and to move this lens group along the optical axis, a diopter adjusting function is additionally provided.
Above the upper limit of the condition (4), an amount of movement of the negative lens group becomes great so that the effect of adjusting the diopter by moving the negative lens in the second lens group becomes poor. On the other hand, below the lower limit of the condition (4), the refracting power of a lens which is moved for adjusting the diopter becomes great, so that a fluctuation in aberration due to a mechanical backlash becomes unfavorably great. As a result, in order to adjust the diopter by moving the negative lens in the second lens group, it is preferable to satisfy the condition (4) that 0.5 less than |f2n |/fe  less than 1 More preferably, the upper limit of the condition (4) should be set to 0.5 or around in order to reduce a Petzval sum.
Furthermore, according to the present invention, it is preferable to introduce a condenser lens between the imaging surface and the erect system and to satisfy the following condition (6):
1 less than fc/fe less than 3xe2x80x83xe2x80x83(6)
where fc indicates the focal length of the condenser lens, and fe indicates the focal length of the entire eyepiece lens at xe2x88x921 dpt, respectively.
In this manner, it is possible to enhance the magnification of the entire eyepiece lens by using the refracting power of the condenser lens itself while, at the same time, correcting the distortion.
Above the upper limit of the condition (6), there arises little effect for enhancing the magnification of the entire eyepiece lens. On the contrary, below the lower limit of the condition (6), it becomes difficult to correct the distortion.
Also, according to the present invention, there is provided an eyepiece lens in an eyepiece lens optical system for observing an image of an object through an erect system which comprises a first lens group having a positive refracting power, a second lens group having a negative refracting power, and a third lens group having a positive refracting power in this order from the object side, and which is characterized in that at least one lens group out of the above lens groups having the positive refracting power satisfies the following condition:
Dp/fe greater than 0.1xe2x80x83xe2x80x83(7)
where Dp indicates the thickness of the lens group having the positive refracting power, and fe indicates the focal length of the eyepiece lens when the diopter is xe2x88x921 dpt.
According to the present invention, it is possible to obtain the effect that an optical path length is prolonged by making at least one of the above positive lens groups thick, so as to enhance the finder magnification, thereby enlarging the distance between the erect system and the eyepoint.
Below the lower limit of the condition (7), it is difficult to obtain the above effect. However, if the lens is made to be too thick, the weight or the cost of the cost is unfavorably increased. Accordingly, it is practically preferable that the lens group is within the condition of 0.3 greater than Dp/fe greater than 0.1 Particularly, when the positive lens group is formed of a single lens, it is preferable that the lens is practically within the condition of 0.2  greater than Dp/fe greater than 0.1 It is also preferable if all of the positive lens groups for constituting the eyepiece lens satisfy the condition of Dp/fe  greater than 0. 1 since the effect of the present invention can be exhibited more excellently.
Also according to the present invention, it is preferable to satisfy the following condition (8):
Dt/fe greater than 0.2xe2x80x83xe2x80x83(8)
where the entire thickness of the eyepiece lens is indicated by Dt on the condition that the above condition (7) is satisfied.
It is possible to enlarge the distance between the erect system and the eyepoint by satisfying the condition (8). Below the lower limit of the condition (8), the total length of the erect system is enlarged, so that it becomes difficult to enhance the finder magnification. Also, since the eyepiece lens becomes thinner, the distance between the erect system and the eyepoint can not be enlarged, so that the object of the present invention can not be achieved. More preferably, the condition of 0. 6  greater than Dt/fe greater than 0.3 should be satisfied. Above this upper limit of 0.6, the erect system becomes thin so that it is difficult to constitute the finder itself.
Also, according to the present invention, it is preferable that the positive lens group which satisfies the above condition (7) comprises a lens having a meniscus shape with the convex surface directed to the object side. With such shape, the exit pupil of the positive lens can be brought closer to the eyepoint As a result, it is possible to see an image in an enlarged manner and to enhance the finder magnification.
Also, according to the present invention, the positive meniscus shape preferably satisfies the following condition:
1.3 less than SF less than 2xe2x80x83xe2x80x83(9)
where SF indicates the same shape factor as that described above.
By satisfying the condition (9), it is possible to easily correct a coma and an astigmatism, so that an excellent aberration state can be securely obtained while maintaining high finder magnification. Above the upper limit of the condition (9), the exit pupil approaches closer to the eyepoint, which is preferable for enhancing the finder magnification. However, in this case, a fluctuation of a coma or distortion which is caused by the diopter adjustment can not be correct. On the contrary, below the lower limit of the condition (9), a chance of occurring an aberration due to the diopter adjustment is reduced. However, the exit pupil is distant from the eyepoint and it becomes difficult to enhance the finder magnification.
When all of the positive lens groups for constituting the eyepiece lens satisfy the conditions (7) and (9), the effect of the present invention is preferably enhanced. It is needless to say that the effect of the present invention can be obtained also when the conditions (7), (8) and (9) are simultaneously satisfied.
Then, it is also preferable to introduce an aspherical surface into an eyepiece lens according to the present invention. Particularly, a distortion can be improved when the aspherical surface is introduced into the positive lens group, while a fluctuation in coma in the range of diopter change can be easily reduced when the aspherical surface is introduced into the negative lens group. Further, it is effective to employ resin material for the eyepiece lens according to the present invention.
When a resin material is employed, the aspherical surface can be introduced more easily because of the lower cost thereof, whereby enabling a mass production of the eyepiece lens.
Moreover, it is preferable to constitute a positive lens for satisfying the condition (7) with a cemented lens. If glasses having different rates of dispersion are used to constitute the cemented lens, it becomes easier to correct a longitudinal chromatic aberration and a chromatic aberration of the magnification. It is also possible to reduce the processing cost by constituting the cemented lens with a single glass.